Background

Using Ruby, Active Record, Sinatra, and a React frontend, I created a traveler tracker for travelers in a country at a given time. Users can scroll through a list of all countries, see the number of travelers in that country, select the country for a list of current travelers, and see the individual visits for the traveler in that country. Additionally, users can see the traveler profile, and there are options to add a new traveler or a new visit for an existing traveler:

The application makes use of three backend models, Country, Visit, and Traveler. A given country has many travelers, through the visits recorded in that country. Similarly, a traveler has many countries, through visits recorded in each of the countries they visited. Each visit belongs to both a country and a traveler (many-to-many association), and only one can traveler can stay at an an accommodation at a time (which is recorded as a visit):

class Country < ActiveRecord::Base

    has_many :visits
    has_many :travelers, through: :visits

class Traveler < ActiveRecord::Base 

    has_many :visits
    has_many :countries, through: :visits

class Visit < ActiveRecord::Base
    belongs_to :country
    belongs_to :traveler

Entity Relationship Diagram

When I was thinking of ideas for the relationships needed to build this application, I originally came up with this ERD:

The Visits are established correctly as the join table, but the Travelers has an unnecessary column as current_country_id, with no association to a country established other than the ID. However, each traveler will have a list of countries they have visited, and you could find the current country of a traveler by just calling #countries[-1] on an instance of a traveler, as such:

class Traveler < ActiveRecord::Base 

    has_many :visits
    has_many :countries, through: :visits

    def current_country
        self.countries[-1]
    end

'from_country_id' is a separate column, and this is used for nationality. I kept this an integer to correspond with the country IDs of the countries table, instead of having to match a nationality string exactly with the country name. We could just find a traveler's nationality by calling #find(#traveler.from_country_id) on an instance of a country. This leaves us with the following ERD:

Finding travelers currently in a country

While we could easily query which country a given traveler is currently in, it is a bit more difficult to find all the travelers that are currently in a given country. Calling #travelers on an instance of a country will return all the travelers that were in that country ever, through the series of visits belonging to each traveler in that country. Similarly, calling #visits on an instance of a country will return all the visits in that country ever, not necessarily the visits of the travelers currently in that country. We can use the #current_country method defined above to help with this.

As calling #travelers on an instance of a country returns all travelers in that country ever, we can iterate through that array of travelers to see if their last country_id corresponds with the country_id of the country instance. If this is the case, we can push those travelers to a new array, and return that array to get all the travelers currently in that country:

class Country < ActiveRecord::Base

    has_many :visits
    has_many :travelers, through: :visits

    def travelers_currently_in_country
        travelers_in_country = []
        all_travelers = self.travelers
        all_travelers.each do |traveler|
            current_country = traveler.current_country
            if current_country.id == self.id
                travelers_in_country << traveler
            end
        end
        travelers_in_country.uniq
    end

How is this used in the application controller?

When I click on a country in the application, we want the travelers in the country to display immediately below the list of countries. I have defined this route in the application controller for get requests to receive JSON of the travelers in a given country, using the above #travelers_currently_in_country method:

get '/travelers_in_country/:id' do
    selected_country = Country.find(params[:id])
    selected_country.travelers_currently_in_country.to_json
  end

We make use of the params hash here, so that the route is dynamic and accepts a country ID. The params hash is created when a request is made to that endpoint. As we are not sending any data in a post or patch request, the dynamic id is the only thing that we need to use with the params hash. We use Country.find(params[:id]) to find the matching country in the database, and then call the #travelers_currently_in_country method on that country.

In the frontend, the fetch request looks like this:

fetch(`http://localhost:9292/travelers_in_country/${selectedCountry}`)
            .then((r) => r.json())
            .then(travelersInCountry => setTravelersInCountry(travelersInCountry))

We pass the ID of the selected country in the url, the application receives the response from the API, and we set state to have the list of traveler's re-render in the frontend. We get an array of objects in this response, and we can then call .map to return JSX to display the travelers in that country.